German Published Patent Application No. 101 48 217.5 describes an injector for injecting fuel whose valve needle is connected to a piezo-actuator. When a voltage is applied to the piezo-actuator, the latter undergoes a change in length that it transmits to the valve needle. This lifts the needle from its valve seat so that fuel can be injected under high pressure from the injector into the combustion chamber of the internal combustion engine.
To drive the piezo-actuator, a setpoint value is generated, which not only is dependent on the desired fuel mass or volume to be injected, but in relation to which other influencing variables that could corrupt the setpoint value must also be taken into account. Influencing variables of this type are, for example, injector temperature or ageing or manufacturing tolerance or similar factors.
The fact that the capacitance of the piezo-actuator must be taken into account when determining the current applied to the piezo-actuator is known from German Published Patent Application No. 198 54 789.7. This is intended to allow the piezo-actuator to be controlled more precisely.